In general, as the welded joints used for buildings, shipbuildings, bridges, construction machines, offshore structures, and other welded structures, there are the multi-layer welded joints obtained by welding by a large number of passes, the fillet welded joints for welding corners by perpendicularly arranging steel plates with each other, and the one-pass or several-pass welded joints obtained by large heat input welding. A detailed description will be given of the conventional problems concerning the toughness of the heat affected zones of the welded joints as described below.
<Multi-Layer Welded Joints>
In a multi-layer welded joint, the microstructure of the steel plate coarsens due to the heat input at welding. However, by the subsequent welding pass, the region where the coarsened crystal grains is heated again, so the crystal grains become finer. Therefore, a high toughness is secured also in the heat affected zone (HAZ, same below). However, the HAZ formed by a last pass near the surface of the steel plate is not subjected to a subsequent welding pass, therefore the crystal grains are not made finer by re-heating. The crystal grains remain coarse as they are, so the fracture toughness is greatly degraded.
For example, Kinzoku Binran (Metal Handbook), edited by the Japan Institute of Metals (revised fifth version), Maruzen Ltd., p. 1072, FIGS. 16 to 50, discloses regarding the microstructure of a welding heat affected zone, that the microstructure once completely-becomes austenite by heating, which thus becomes extremely coarse, apt to harden and crack.
In order to solve this, conventionally, the subsequent welding has been performed even after the predetermined welding is finished, so that the related portion is welded until a built up state to increase the refined-grains in the microstructure. Then, the unrequired excess buildup is ground away by a grinder etc. so as to thereby allow only the HAZ increased in grain refinement by re-heating to remain. By such a measure, however, excess welding work becomes necessary such as grinding work etc. Therefore, there was a problem that the installation costs and a process load were large, so this was not realistic.
<Fillet Welded Joints>
In a fillet welded joint for welding a corner by perpendicularly arranging steel plates with each other as well, the crystal grains coarsens in the HAZ vicinity of a toe portion. Therefore, there was the same problem as that of a multi-layer welded joint.
<Large Heat Input Welded Joint>
A steel plate to which a one-pass or several-pass welded joint obtained by large heat input welding is applied is generally designed to prevent the coarsening of the HAZ microstructure even without repeated heat input by a subsequent welding pass by increasing the refinement and dispersing TiN, oxides, etc. in the steel plate matrix and thereby enabling suppression of the austenite grain growth by the pinning effect of TiN, oxides, etc. However, the HAZ microstructure easily become coarse when the amount of heat input of the welding is large. Therefore, if restricting the amount of heat input or improving the welding efficiency by making the groove narrower and nearer to vertical, the steel plate matrix is not sufficiently melted and the weld metal ends up solidifying earlier. Therefore, undercut in welding easily occurs at the toe portion. This zone becomes a stress concentration site and becomes a fracture initiation point, so there arises a problem of a remarkable drop in the fracture toughness.
Further, as prior art relating to the method of imparting ultrasonic vibration to the welding zone, for example, U.S. Pat. No. 6,171,415 discloses a method of imparting ultrasonic vibration along a welding seam heated by a welding arc immediately after arc welding. However, this prior art is a method of improving the fatigue strength by impacts by an ultrasonic vibrator and does not discloses anything about increasing the refinement of the microstructure of the HAZ or improving the toughness by pressing together the poorly fused parts of the object of the present invention.